The present invention relates to a method for joining together two or more metal articles by brazing. More particularly, the present invention relates to methods for depositing a flux material with or without metal powders onto a metal substrate prior to a brazing operation.
Aluminum and its alloys are particularly useful materials for inclusion in metal components of vehicles such as cars, trucks, airplanes, and the like. Aluminum alloys are lighter than steel alloys and thus offer weight advantages in many applications in vehicles. The light weight and excellent heat transfer properties of aluminum alloys make them particularly attractive candidates for use in heat exchangers such as radiators, heaters, evaporators, oil coolers, condensers and the like. These heat exchangers and similar components are typically fabricated from a multitude of formed or extruded parts that are subsequently assembled, fixtured, cleaned and joined together in a brazing process. In brazing of aluminum work pieces, an aluminum brazing alloy (e.g., an aluminum-silicon alloy) is positioned between the surfaces to be joined and the work pieces are heated to a temperature which melts the brazing alloy but not the underlying work piece. Upon cooling, the brazing alloy solidifies as a joint between the work pieces. The brazing alloy is typically introduced onto the surfaces of aluminum stock by cladding thereto in a roll bonding operation.
A common brazing practice includes cleaning of the components via a suitable solvent (to remove oils and the like from the surfaces to be brazed) followed by application of a flux to the pre-brazed components to be joined. The fluxed components are heated in a controlled atmosphere to retard oxidation, this atmosphere being typically dry nitrogen. The role of the flux is to reduce the oxides on the faying surfaces of the components which are to be joined via brazing. The flux is applied after fabrication of the individual work pieces to be brazed, commonly after assembly of the components (e.g. as a heat exchanger) prior to brazing. The flux may be applied directly as a dry powder or mixed with a carrier such as water or alcohol and applied as a slurry over the entire work piece. In the latter case, the carrier is subsequently removed via a drying step, leaving the flux as a powder on the surface of the work piece.
The flux is only required in areas where metallurgical bonds or joints are required. Nevertheless, it is common manufacturing practice to apply flux over the entire assembly, often including the fixtures used to contain the parts during the brazing step in the furnace. This results in overuse and waste of flux, the need to clean the fixtures and increased maintenance of the furnace due to the corrosive nature of flux. Moreover, the processes of cleaning and applying flux are time consuming and concomitantly expensive. It should be further noted that the flux is loosely adhered to the work pieces as a powder. Hence, care must be taken to avoid removal of the flux during any handling of the components prior to brazing.
An alternative to fluxing the entire assembly is to apply flux to the work pieces prior to working or forming the material in a pre-fluxing operation. Pre-fluxing is advantageous in that the flux can be applied only on the cladding where joints are formed; unclad areas are without flux. However, conventional pre-fluxing techniques have not found broad commercial applications.
One pre-fluxing method has been to disperse flux in a binder and coat the work piece with the flux-binder mixture. During brazing, the binder volatilizes which may results in undesirable voids within the joint that must be filled to ensure sealing of the brazed components. Another drawback to this flux-binder coating technique is that the brazing surfaces typically must be cleaned beyond standard rolling mill cleanliness standards thereby increasing the operating costs by several cents per pound of brazing metal produced.
An alternative route to pre-fluxing is to eliminate the cladding process and apply flux and a cladding metal or alloy in deposition processes either simultaneously or sequentially. One such technique is thermal spraying as disclosed in U.S. Pat. No. 5,594,930. The ""930 patent teaches spraying molten droplets of aluminum and silicon or an alloy thereof onto a brazeable aluminum substrate. U.S. Pat. No. 5,820,939 also discloses a method of thermally spraying metallic coatings on unroughened cleaned aluminum alloy substrates. The method includes wire-arc thermally spraying of melted metallic bonding droplets and fluxing particles onto the substrate using gas propulsion to concurrently deposit flux particles and bonding droplets. In these methods, molten droplets pass through air and form additional oxides thereon which compounds the need to deoxidize the substrate.
Hot pressing of powders of aluminum, silicon or an alloy or mixture thereof onto an unclad aluminum substrate is described in U.S. Pat. Nos. 5,330,090 and 5,547,517. Compaction of powders typically results in minimum void levels of about ten percent. Voiding is undesirable and the process of hot pressing the powders onto the substrate can be cumbersome.
Coating processes for simultaneous application of flux with aluminum and silicon are described in U.S. Pat. Nos. 5,100,048 and 5,190,596. The ""048 patent teaches a process of dipping unclad aluminum substrate into an alcohol slurry of aluminum, silicon and flux. Upon evaporation of the alcohol, the silicon and flux remaining on the substrate is weakly adhered thereto and tends to spall off the substrate during assembly. The ""596 patent discloses a method of applying a paste containing aluminum, silicon and a binder onto unclad aluminum substrate. In either case, the silicon and aluminum form a thin clad layer on the aluminum substrate and a flux is incorporated therewith. This system adheres better to the substrate, but the volatilized binder creates voids in the joint.
Accordingly, a need remains for a method of depositing brazing flux onto metal substrates prior to working of the metal which minimizes the amount of flux used in the brazed assembly, adheres flux to the substrate without the use of a binder, and may additionally deposit metal cladding into the substrates.
This need is met by the method of the present invention which includes a method of treating a surface of a metal article, a metal substrate, by spraying a treating composition including metal halide particles dispersed in a carrier gas onto a surface of a metal article at a sufficiently high velocity to form a layer of the metal halide particles on the surface. The minimum velocity for deposition of the metal halide particles is about 100 m/sec. This technique is particularly useful for pre-fluxing brazing components. The gas may be air, helium, nitrogen or combinations thereof and may have a temperature of about room temperature to about 500xc2x0 C. The type of gas and the treating composition temperature may be varied to control the velocities of the particles entrained in the gas of the treating composition. Less dense gas (e.g. helium), higher temperatures and higher pressures provide higher particle velocities.
Another set of particles, preferably formed from a metal, an alloy thereof or a mechanical mixture of a metal and an alloy thereof, may also be dispersed in the gas. Hereinafter, reference to a metal as the material of a substrate, particle or coating is meant to include the metal, alloys thereof as well as mechanical mixtures of metals and metal alloys unless otherwise indicated. The metal or metal alloy particles are believed to assist in deposition of the metal halide particles onto the surface of the metal article. The metal halide particles and the metal particles preferably are each about 5 to about 50 xcexcm in diameter. The velocity of the particles sprayed onto the surface of the metal article being treated determines whether the metal halide particles alone are deposited onto the surface or whether the metal halide particles and the metal particles are co-deposited onto the surface. In one embodiment, the velocity of the particles is selected so that only the metal halide particles are incorporated into the surface of the article while the metal particles recoil or bounce off from the surface and are not incorporated into the article. When the treating composition is sprayed at velocities of about 200 to about 550 m/sec, a layer of metal halide particles is deposited onto the metal surface in the amount of about 1 to about 12 grams per square meter of the surface.
In another embodiment, the treating composition is sprayed at a velocity whereby both of the metal halide particles and the metal particles are incorporated into the surface of the article. A higher velocity of the treating composition is needed than for incorporating only the metal halide particles into the article surface which preferably is over about 550 m/sec. This embodiment results in a layer of metal halide on the surface of the metal article and also creates a clad layer of the metal particles.
The method of the present invention may be used to treat metal articles formed from aluminum alloys, copper alloys, steel alloys, magnesium alloys, and nickel alloys. Suitable aluminum alloys are those of the Aluminum Association 1xxx, 2xxx, 3xxx, 4xxx, 5xxx, 6xxx, 7xxx or 8xxx series. The present invention is particularly suited for producing pre-fluxed brazing sheet which is either clad or unclad. Unclad brazing sheet may be fluxed and clad in a single process using the method of the present invention.
In yet another embodiment of the invention, the metal halide particles may be encapsulated with a metal such as Al, Cu, Zn, Mg, M, Ni, In, Li or Fe. The metal coating over the metal halide particles provides for a metal-to-metal adhesion of the encapsulated particles to the substrate. Other particles, including those which otherwise traditionally exhibit poor adhesion to metal substrates, such as particles of a transition metal (e.g. silicon or silicon alloys), may be encapsulated in these metals and may be deposited as well. These encapsulated particles provide an opportunity to apply flux and a clad layer to brazing sheet with superior adhesion properties.
The patent or application file contains at least one drawing executed in color. Copies of the patent or patent application publication with color drawings will be provided by the U.S. Patent and Trademark Office upon request and payment of the necessary fee.
Other features of the present invention will be further described in the following related description of the preferred embodiments which is to be considered together with the accompanying drawings wherein like figures refer to like parts and further wherein:
FIG. 1 is a ten times magnified photomicrograph of the aluminum coupon coated in Example 1;
FIG. 2 is a ten times magnified photomicrograph of the aluminum coupon coated in Example 1 after working;
FIG. 3 is a back scattered electron image showing a cross section of the aluminum coupon coated in Example 2;
FIG. 4 is an x-ray map of the image of FIG. 3 showing the location and concentration of the element aluminum
FIG. 5 is an x-ray map of the image of FIG. 3 showing the location and concentration of the element silicon;
FIG. 6 is an x-ray map of the image of FIG. 3 showing the location and concentration of the element potassium; and
FIG. 7 is an x-ray map of the image of FIG. 3 showing the location and concentration of the element fluorine.